Clarification of rayon spin baths



Patented Apr. 4, 1939 UNITED STATES PATENT FFECE CLARIFICATION OF RAYON SPIN BATES No Drawing. Application July 7, 1937,

Serial No. 152,469

4 Claims.

This invention relates to a process for the clarification of fouled spin baths used in the viscose rayon manufacture.

In the spinning of rayon the cellulose xanthate 5 viscose is extruded through fine openings in socalled spinnerettes into a bath which regenerates hydrated cellulose to form a filament which maythen be spun with others to form rayon yarn. These spin baths contain salts and acids for delo composing the strongly alkaline cellulose xanthate. Typical baths contain from 8-10% of sulfuric acid, glucose and various salts such as sodium sulfate, sodium sulfite and the like. During spinning the alkali in the viscose reacts to form 15 sodium sulfate and the decomposition of the xanthic acid radical results in setting free a considerable amount of sulfur which is present in a very finely divided and sometimes colloidal form. The bath soon fouls with decomposition products including sulfur and requires clarification before the bath can be reused. The volume of spin baths used is very large, one-half liter of spin bath per minute per spindle being a common amount in the rayon industry. With the large volume of bath required it is of course economically undesirable to discard the fouled solution as it contains too large a quantity of valuable chemicals.

Various methods of the clarification of the spin baths have therefore been proposed and some are in fairly extensive use. Most of the methods have depended on filtration using, for example, excelsior, wood, shavings, burlap, coarse woolen cloth and the like. A certain degree of clarification is achieved by these filters but they are only used where a high degree of clarity of bath is not required. In recent years some operators have required a bath of a higher degree of clarity and have installed sand filters which operate continuously. These filters, however, require periodic backwashing with water and considerable losses of bath occur. While it is possible to effect clarification by the prior filtering processes, they are expensive and the highest clarity is only obtained with special filters requiring very careful supervision and even with the best filters clarity is not as high as could be desired. The filtering processes also do not permit an economic recovery of the sulfur in a form in 60 which it is readily salable.

According to the present invention, spin baths are clarified by froth flotation such as is used in the ore dressing art. It is found that the sulfur and other impurities can be floated off from the bath with a very slight loss of bath and with the production of an extraordinarily clear bath for reuse.

The flotation process is in some respect unique in that it is apparently not dependent on the use of any particular definite flotation agent. It is possible to fioat with air alone without any reagents or with the addition of frothers or collectors. This is most unusual as the ordinary flotation processes are rather rigidly limited to the use of particular classes of collecting agents.

The great flexibility of the process of the present invention is one of its important advantages. While a good clarified concentrate with fairly good recovery is obtainable without any reagents, materially better sulfur recovery can be ob ained by the use of r. hydrocarbon collector such as kerosene, whereas substantially the same recovery as is obtained without reagents results when flotation agents of the dithiophosphate type are used. These reagents, however, give a purer concentrate.

The question of whether a reagent should be used is largely one of economics because, while the process without reagents gives good recovery and good clarification, this is obtained at the expense of lower flotation machine capacity. In every case, of course, the best economic compromise will be chosen between additional reagent cost and saving in equipment and power charges. The process of the present invention is also unique inanother respect; namely, that the same fiotation procedure removes a concentrate which contains sulfur to the extent of '70- 80% with the remaining material which is chemically totally unrelated to sulfur. Such a result is in marked contrast to the ordinary flotation processes which are highly selective and which depend for their action on the separation of materials of different types of chemical composition. The reason for the anomalous behavior of the present process is not as yet determined, and the invention is not limited to any set theory of operation.

The present invention is not limited to any particular type of flotation machine. Excellent results are obtained with highly efiicient machines of the mechanical type such as a Fagergren machine. However, the present invention represents a peculiar situation because simple pneumatic machines can be used without serious 1055 in flotation efficiency. This is not the case with most flotation processes where the highly developed mechanical machines with their superior agitation and aerating efliciency have relegated the pneumatic machines to a relatively minor position in the art. The possibility of using extremely simple pneumatic machines in the present process is of great importance because the spin baths are strongly acid and hence any metallic equipment has to be rendered acid proof, which is a difllcult matter with mechanical machines having rotating parts. Pneumatic machines can of course be made of completely acid proof construction without any materially increased cost.

The invention will be described in greater detail in conjuncticn with the following specific examples which set forth the clarification of typical spin baths using various reagent combinations. It should be understood that the invention is not limited to the particular details set forth in the examples.

Example 1 A fouled spin bath having a composition typical of that used in bobbin spinning and containing when fresh 130 grams sulfuric acid per liter, 250 grams sodium sulfate per liter and 14 grams zinc sulfate per liter was treated in an acid proof mechanical flotation machine of the Fagergren type. The bath was clarified by three methods, first with the use of no reagents, then with a dicresyldithiophosphoric acid known in the trade as Aerofloat 15, then with an alcohol frother containing a mixture of higher alcohols and known in the trade as 13-48, and finally with kerosene. In every case, the bath was thoroughly clarified and a good recovery of sulfur was obtained. The details of the tests appear in the following table. The volumes and weight parts given in the table are in the same system:

the results were substantially similar but with slightly lower recovery percent.

Example 3 A spin bath typical of the pot spinning process was floated in a pneumatic machine as described in Example 2. The results with no reagents were substantially the same as the corresponding results in Example 1 and in a second test, when the reagent was Aerofloat 15, a slightly better grade of concentrate was obtained with a larger output from a given size machine.

I claim:

1. A method of clarifying fouled regenerating,

baths from the regeneration of hydrated cellulose from viscose which bath contains sulphur deposited out of solution, which comprises subjecting the bath to froth flotation and removing the froth which contains the bulk of the turbid impurities.

2. A method of clarifying fouled regenerating baths from the regeneration of hydrated cellulose from viscose which bath contains sulphur deposited out of solution, which comprises subjecting the bath to froth flotation in the absence of flotation reagents and removing the froth which contains the bulk of the turbid impurities.

3. A method of clarifying fouled regenerating baths from the regeneration of hydrated cellulose from viscose which bath contains sulphur deposited out of solution, which comprises subjecting the bath to froth flotation in the presence of a dithiophosphoric acid compound, and removing the froth which contains the bulk of the turbid impurities.

4. A method of clarifying fouled regenerating baths from the regeneration of hydrated cellu- Reagent Spin bath Impurities removed Impurity Percent Type Lgblilgn w gs congeilif, P ts by 01 mm Parts lbty Recoverty, figigg par S y V 111116 V0 ume welg DGI'CBD bath treated weight treated percent None None 1, 669 1. 442 170 10.2 1.348 93.48 78.1 0. 105 2, 240 0. 484 230 10. 3 0. 450 02. 0s 79. 0 -4s 0. 11s 2, 230 o. 482 222 10. 0 0. 415 80. 7s. 0 Kerosene 0.172 1,900 0.410 204 10.7 0.402 98.05 74.0

Example 2 lose from viscose which bath contains sulphur A spin bathas described in Example 1 was treated in a pneumatic flotation machine provided with an acid proof air distributor in the bottom and of acid proof construction throughout. The results were substantially the same as in Example 1.

When cresylic acid was used in place of 3-48,

deposited out of solution, which comprises subjecting the bath to froth flotation in the presence of a saturated hydrocarbon collector and removing the froth which contains the bulk of the turbid impurities.

ROBERT IB. BOOTH. 

